1. Technical Field
The present invention relates to separation of impurities from liquids and, more particularly, to the use of electric fields to effect separation of impurity particles from low dielectric liquids having ultra-low electrical conductivity. An example of such a liquid is oil.
2. Discussion of the Prior Art
Conventional solid-liquid separation methods, such as barrier or deep bed filtration, gravitational settling, centrifugation and rotary drum precoat filtration, are generally ineffective or prohibitively expensive for use in separating small particles from from viscous fluids such as low conductivity liquids. This is especially true for applications involving very small particles or highly viscous liquids because conventional methods depend upon gravitational force, particle inertia, or fluid forces; in some cases, conventional techniques require that the liquid be forced through porous filtration media.
Separation techniques based on electrical forces have particular applicability to problems involving small particles and high viscosity liquids. This is true because the electrical force acts primarily on the particles, and also because the electrical force can be significantly larger than gravitational and fluid forces. Two electrical transport mechanisms, namely dielectrophoresis and electrophoresis, have been suggested in the prior art as a basis for particle separation technology.
Dielectrophoresis is the transport of an uncharged, polarizable particle in a nonuniform electric field. Dielectrophoresis forms the basis of the particle separator disclosed in U.S. Pat. No. 3,928,158 to Fritsche. The separator device in Fritsche was developed to remove catalyst fines from decanted oil to produce a low ash carbon black feedstock. The device includes concentric cylindrical electrodes with ceramic or glass beads filling the inter-electrode space. A large potential difference is applied to the electrodes so that the high gradient in the electric field existing near the points of contact of the glass beads effects particle removal.
Electrophoresis is the transport of a charged particle in an electric field. Particles suspended in aqueous systems often carry a net charge, and electrophoresis takes advantage of this fact and is widely utilized in bio-medical applications and has also been suggested for aqueous systems. However, the aqueous applications of electrical filtration are severely limited by high power consumption even at low electric fields. In dielectric liquids, on the other hand, it is possible to use much higher fields (for example, on the order of 10.sup.6 to 10.sup.7 volts per meter). Although particles present in some hydrocarbon liquids have been found to carry a net charge, the particle charge levels are often too low for effective filtration. For this reason, the cross-flow electrofiltration concept has been modified in some prior art systems to render it applicable to low conductivity liquids by adding a surfactant to the liquid for the purpose of enhancing particle charge.
Prior to the present invention, all electrokinetic methods for separating particles from liquids depend upon chemical additives for particle charge acquisition. The principal mechanism is the preferential adsorption of either positive or negative ions in solution. In low conductivity liquids the concentration of these ions (referred to herein as intrinsic ions) is often too low for the particle charge acquisition to be significant. For this reason, as noted above, surfactants have been added to the liquids. This increases liquid conductivity and can increase charge levels. These enhanced charge levels are still too low for economical particle separation via electrical methods, due to the typical high viscosity of oils. Also, due to increase of liquid conductivity, the electrical power consumption also increases with the addition of ionic chemicals.
For definitional purposes, intrinsic ions are those attributable to the inherent conductivity of the liquid, dissociated electrolytes or other chemical processes. Ions produced by electrical discharge or by ionizing radiation, or those emitted from an electrode, are referred to as extrinsix ions. The present invention relates to the use of extrinsic ions.